A DPF (diesel particulate filter) that traps particulate matter (PM) contained in exhaust gas and an LNT (LNT occlusion reduction catalyst) that reduces and purifies NOx are used in a diesel engine (internal combustion engine) as an exhaust gas post-processing device. Further, an in-exhaust pipe injection, in which fuel is injected directly into an exhaust pipe, is known as a method of forcibly regenerating the DPF and the LNT. An in-exhaust pipe injection is advantaged over a post injection in that oil dilution by the injected fuel does not occur. Further, EGR can be performed in conjunction with the forcible regeneration of the exhaust gas post-processing device, and therefore NOx deterioration and the like can be prevented.
The fuel injected during the in-exhaust pipe injection reaches the downstream side DPF and LNT while being thermally decomposed (vaporized), raises the temperatures of the respective catalysts through combustion of HC and CO, and thus functions as a NOx reducing agent. A temperature at which thermal decomposition of the injected fuel into HC and CO begins is no lower than 200° C. to 250° C., and therefore, to prevent the fuel from adhering to the exhaust pipe and so on, the in-exhaust pipe injection is preferably performed when an exhaust gas temperature is no lower than 200° C. to 250° C. Hence, when the in-exhaust pipe injection is performed at an exhaust gas temperature equal to or lower than 200° C., for example during idling, a multistage injection must be performed concurrently in order to raise the temperature of the exhaust gas.
Japanese Patent Application Publication No. 2007-170218, for example, discloses an exhaust gas purification device in which a NOx occlusion reduction catalyst is provided in an exhaust passage of an internal combustion engine, and which is capable of performing an in-exhaust pipe injection and a multistage injection concurrently during NOx reduction control.